A joint is where two bones meet. Synovial joints are one of three types of joints in the body. Synovial joints are unique because they contain a fibrous joint capsule with synovial fluid. Hinge and ball-and-socket joints are both types of synovial joints. Saddle joints are another type of synovial joint.

The saddle joint gets its name because the bone forming one part of the joint is concave (turned inward) at one end and looks like a saddle. The other bone’s end is convex (turned outward), and looks like a rider in a saddle.

Saddle joints are also known as sellar joints. These highly flexible joints are found in various places in the body, including the thumb, shoulder, and inner ear.

Unlike hinge joints, such as those between the bones in your fingers, saddle joints have a much greater range of motion than a simple backward-and-forward movement. Saddle joints have two basic types of movement, known as flexion-extension and abduction-adduction.

Flexion and extension are opposite movements, but they’re easy to visualize. When you bend your elbow, you decrease the angle between your upper arm and your forearm. This is an example of flexion. When you straighten your arm, you’re extending it, increasing the angle between your upper and lower arms. This is an example of extension.

Abduction and adduction are movements related to the midline of a structure. For example, spreading your fingers wide moves them away from the midline down the center of your hand. Adduction is a return toward the midline.

Some saddle joints are also capable of combining flexion-extension and abduction-adduction movements.

Trapeziometacarpal joint

The prime example of a saddle joint is the trapeziometacarpal joint at the base of your thumb. It connects the trapezium and the metacarpal bone of your thumb.

The flexion-extension and abduction-adduction characteristics of this joint allow your thumb to spread out wide to help grasp large objects, while also allowing it to move inward, to tightly touch each of your other fingers.

This is also a fairly common site for osteoarthritis, which can cause pain, weakness, and stiffness in your thumb and inner wrist.

Use this interactive 3-D diagram to explore the trapeziometacarpal joint.

Sternoclavicular joint

This joint is where your clavicle (collarbone) meets your manubrium, which is the upper part of your sternum (breastbone). It allows you to raise your arm over your head, among other movements, and also supports your shoulder.

Use this interactive 3-D diagram to explore the sternoclavicular joint.

The ligaments that surround this joint are some of the strongest in your body, which make the sternoclavicular joint hard to injure. However, high-impact collisions, falls, or car accidents can all damage your sternoclavicular joint.

Incudomalleolar joint

This joint is located in your inner ear, where it connects two small bones called the malleus and incus. They’re both vital to your ability to hear. The incudomalleolar joint’s main function is to help transfer vibrations in your ear, which are perceived as sounds by your brain.

Use this interactive 3-D diagram to explore the incudomalleolar joint.

Head injuries, long-term ear infections, and inserting foreign objects too far into your ear can all cause damage to this joint and affect your hearing.

You don’t have many saddle joints in your body. However, the ones you do have are crucial to many daily activities, from listening to music to grasping things in your hand.